Another physlet that comes with neat features you can access by setting ShowControls to true is the electric field physlet, which allows you to add charged objects and visualize the resulting electric field.

To make the electric field physlet work on your own machine, you will need to download the EField4_.jar file from the Physlets site at Davidson College. Select the "Download" link, and then select the "JAR FILES" link.

Right-click on the EField4_.jar file and save it to the "classes" folder.

To add something such as a lens or an object to the simulation, click the appropriate button above or below the simulation and then click inside the simulation at the position you'd like to place it.

Click-and-drag anything in the simulation to move it or, in the case of an object, to re-size it.

Clicking on a lens brings up two points, one on either side of the lens. These are the focal points. Click-and-drag either focal point left or right to change the focal length of the lens.

Dragging a focal point through a lens changes the lens from concave to convex or vice versa.